1 The influence of botanical composition on annual phytomass production of
a semi-arid grassland in response to precipitation was tested with a 19-yea
r experiment. Three compositional states reflecting good, medium and poor r
angeland condition, whose basal cover increased from poor through to good,
were created in 1978.
2 Multiple regression models were developed for assessing whether phytomass
was influenced by precipitation, composition, phytomass of the previous ye
ar, basal cover, abundance of individual species, or diversity. Composition
and precipitation accounted for 66% of the variation in phytomass, but a s
eparate model of basal cover by precipitation was equally successful. The l
inear effect of rainfall on phytomass was enhanced as composition improved
from poor (0.076 g m(-2) mm(-1)) through medium (0.197 g m(-2) mm(-1)) to g
ood (0.277 g m(-2) m(-1)). Phytomass increased for low precipitation if it
had been high the preceding year. Phytomass was more variable over time wit
h deteriorating condition.
3 Species' complementarity ensured greater and more stable production. Seta
ria sphacelata, Eragrostis chloromelas and Digitaria eriantha increased phy
tomass on the good or medium condition grasslands during drier years, where
as Themeda triandra had this effect during wetter years.
4 Precipitation-use efficiency (PUE) was influenced mostly by composition a
nd a linear and quadratic effect of precipitation (63% of variance). Optimu
m PUE of 0.308, 0.203 and 0.096 g m(-2) mm(-1) for the good, medium and poo
r condition grasslands, respectively, occurred at intermediate amounts (+/-
680 mm) of precipitation. PUE was increased if phytomass had been high the
5 Species' complementarity of PUE in response to precipitation was evident
for all compositional states. Ten, mostly uncommon, species and their inter
action with precipitation explained an extra 21-42% of the variance. Stabil
ity of production was related to PUE for medium and poor condition grasslan
d. Uncommon species therefore ensured growth efficiency and stabilized prod
uction as condition deteriorated.
6 Diversity had no influence on phytomass or PUE except for a small to mode
rate effect, respectively, for the medium condition grassland.
7 Vegetation structure, through limiting runoff and promoting infiltration,
is an important control on the amount and efficiency of plant production u
nder variable precipitation, whilst composition further influences the amou
nt and stability of production.